This invention pertains to hermetically sealed, positive displacement compressors for compressing refrigerant in refrigeration systems such as air conditioners, refrigerators, and the like. In particular, the invention relates to multi-unit compressor assemblies wherein a plurality of compressors are housed in a common hermetic shell.
Various positive displacement compressors for use in refrigeration systems are known in the art. For example, reciprocating piston, rotary vane, and scroll type compressors have been incorporated into the various hermetic compressor assemblies. Each of these compressor types typically includes an electric motor drivingly connected to a crankshaft having an eccentric portion thereon. During operation of each of the aforementioned compressors, a piston means such as a cylindrical piston, a roller, or an orbiting scroll is operatively driven by the eccentric portion of the rotating crankshaft to compress refrigerant within a compression chamber. In a hermetic compressor assembly having a pressurized or high side sealed housing, the compressed gas from within the compression chamber is discharged into the compressor housing.
Certain refrigeration system applications require a compressor assembly having a greater displacement capacity than is typically available from standard application compressor units. To meet this need for greater capacity, a single positive displacement compressor may be made physically larger, thereby increasing its displacement. Alternatively, it is known to supply a single refrigeration system with a pair of motor-compressor units connected in parallel to the system, whereby either unit may be cycled on and off to vary the capacity of the system. In this latter approach, the two motor-compressor units are of the vertical shaft type and are mounted within a single housing in spaced parallel relationship to one another.
A disadvantage that often results from the prior art solution wherein a single larger compressor is used, is an increased incidence of vibration, noise, and operating inefficiency. In the case of a compressor having a single piston means, the increased physical size results in greater vibration. Increased amplitude noise pulses are produced by a larger capacity compressor as the result of greater mass flow rates of compressed gas through discharge passages. Several factors in a physically larger compressor contribute to operating inefficiencies, one notable factor being the increase in power losses due to increased friction from larger bearings required to support the necessary larger crankshaft. Furthermore, inherently inefficient motor loading for a single piston compressor is further degraded due to the greater operating range required of the motor and the possibility of overload at high end loading.
In order to decrease vibration in a single motor compressor unit, it is known to increase the number of compression chambers and pistons. However, several problems and disadvantages are associated with this approach. For example, in a reciprocating piston type compressor the addition of more pistons and cylinders requires a longer crankshaft having multiple eccentric portions. Besides the added size, complexity, and expense of increasing the number of cylinders, there are problems associated with increased crankshaft length. One such problem is what is known as rotor whip, wherein the rotor of the electric motor exhibits radial variations which cause difficulties in maintaining a proper air gap between the motor, rotor and stator. Also, a greater crankshaft length may require an increased number of bearings, thereby adding further to the expense and complexity of the compressor.
A multi-unit system is often more efficient and less expensive to manufacture and operate than a larger single unit of the same capacity. However, the prior art multi-unit systems remain complex and expensive due to the typical modular design wherein separate independently operating motor-compressor units are mounted within a single housing. In such an arrangement, a large housing is required and very few dimensional efficiencies are gained. This is due primarily to the need for multiple electric motors to drive the multiple compressor units.
In the prior art multi-unit compressor assemblies it is known to provide a single suction accumulator within the hermetically sealed housing. In such an arrangement, either the suction inlet tubing is connected between the suction accumulator and the individual compressor units, or the interior of the housing is at suction pressure, thus obviating the need for tubing connecting the accumulator to the compressor. A disadvantage of having the suction accumulator mounted within the housing is that additional space within the housing is required, as well as additional tubing. Furthermore, the suction accumulator is subjected to the hotter operating environment within the housing.